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Controlling magnetic properties by tuning carrier concentration is a key feature of diluted ferromagnetic semiconductors, giving possibilities for multifunctional spintronic devices. Here, we report a giant Zeeman shift from the spin-polarized state in tungsten diselenide (WSe₂) doped with a small amount of vanadium atoms (~0.15%) by investigating resonant magnetotunneling spectroscopy of a vertical graphite/V-WSe₂/graphite heterojunction. The p-type character of the doping state is located near the valence band, substantially shifted under an external magnetic field at 7.8 meV/T with a giant g factor of approximately 135, an order of magnitude higher than that of other 2D magnetic semiconductors. The evidence of the spin-polarized band edges of V-doped WSe₂ is also confirmed by circular-polarized photoluminescence. A deep understanding of the nature of the V-WSe₂ system provides opportunities for future spintronics based on 2D van der Waals diluted ferromagnetic semiconductors.